The first of three satellites designed to undertake a comprehensive study the Earth’s magnetic field left Munich airport on Tuesday aboard an Ilyushin-76 transport aircraft bound for Plesetsk Cosmodrome, 200 km (124 mi) south of Arkhangelsk, Russia. This first Swarm satellite will be followed by two identical craft and more than 60 tons of support equipment before all three are launched into polar orbit in November.

Most of us know about the Earth’s magnetic field from Christmas cracker compasses and half-forgotten science classes, but there’s more to it than orienteering or piloting a boat. The field, among other things, protects the Earth against cosmic rays, preserves our atmosphere by keeping away the solar winds, and keeps the planet from ending up like Mars.

It’s also more complex than it’s portrayed. Schoolbooks often demonstrate the Earth’s magnetic field by showing the planet with a bar magnet stuck through its axis, but it isn't that simple. The magnetic field is dynamic and mainly the result of the rotation of Earth’s molten core in a manner a bit like a dynamo. Just as a piece of iron left in a magnetic field can become magnetized, the same is true of the minerals in the Earth’s crust, which add their bit to the field, as does the electrical currents in the saltwater of the ocean, and then there are the effects of the Sun.

All of this produces a field with a complex shape where the magnetic lines of force vary from true north, the magnetic poles shift regularly and can even reverse, and where the exact nature of the field in any one place is a complex sum of all sorts of interactions.

And it's not just a matter of academic interest. Disruptions in the geomagnetic field can affect navigation, animal migration, power grids, communications and even computers. It also gives scientists a window into the structure and nature of the Earth’s interior.

This is where Swarm comes in. Built by Astrium, the Swarm constellation is part of the European Space Agency's "Living Planet" program and is the result of over thirty years of work on deep space magnetic research by Astrium, including its work on the Champ and Cryosat satellites.

The three “magnetically clean” satellites will be launched into two polar orbits, with two of them flying side by side at an altitude of 450 km (280 mi) and a third at 520 km (320 mi) in hopes of making the most accurate survey yet of the geomagnetic field.

Each satellite is equipped with an Absolute Scalar Magnetometer, a Vector Field Magnetometer, an Electrical Field Instrument, an accelerometer, a Laser Retro Reflector, a star tracker system and a GPS receiver. Working in concert using highly accurate navigation data, they’ll be able to take high-resolution measurements of the strength, direction and variations of the geomagnetic field as well as distinguishing the various sources of the field.

Scientists hope that these measurements will not only help them to produce better models of the field, but also improve terrestrial navigation and weather forecasting, aid in prospecting for mineral resources, reveal more about the Earth's interior structure and provide better warnings of dangerous solar storms.

Sources: Astrium, ESA

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